Support for GFM task listsbrowsing

I've been loving Editorial's Taskpaper support, and recently had the thought that a very nice alternative/complement to that would be support for a GitHub Flavored Markdown (GFM) extension: Task Lists.

I'm aware of GFM, and I originally wanted to support this in v. 1.2, but it turned out to be slightly more difficult to implement than I anticipated...

In TaskPaper mode, I basically just replace the dash ('-') with a checkbox widget, but it's a little more difficult to do this when there are multiple characters involved (e.g. cursor placement is a little bit of a problem because it could end up "inside" the checkbox).

It's still something I'd like to support at some point, but I don't know when I'll get to it yet.